Amplified sub-nanosecond laser pulses in the mid-IR spectral range (2-15 μm) are increasingly important for new scientific, technological and medical applications. These wavelengths are useful because they are resonant with the vibrational transitions that provide fingerprints that are highly specific to a particular molecule or material. This unique property of absorption in the infrared spectral region imparts important novel properties to high peak power and average power mid-IR lasers. Tuning to match specific vibrations enables the selective excitation of materials as means to selectively process and shape the material through ablation.
Utilization of this phenomena has led to recent advances in the laser processing of materials, which is based on a new understanding of the ablation process and the dynamics of transduction of vibrational energy into heat. This novel method of laser ablation, as detailed in U.S. patent application Ser. No. 11/321,057, filed Dec. 30, 2005, is able to achieve efficient material ablation with the minimum of collateral damage through either ion formation or thermal accumulation. This is accomplished by impulsive heat deposition (IHD), a novel method that combines both thermally and photomechanically driven ablation mechanisms, in which most of the absorbed energy remains in the ablated material [1]. The laser energy is coupled directly to the mechanical degrees of freedom that lead to ablation with optimal efficiency, which is key to minimizing collateral damage. Stated most succinctly, if the material can be energized and this energy is thermalized into heat faster than the material can expand, all the energy becomes stored locally.
As soon as the material or one constituent of the material (e.g. micropools of water in cells) has its temperature raised approximately two times its equilibrium phase transition, the material will undergo an explosive phase transition driven by homogeneous nucleation unique to inertial confinement. The ensuing volume changes and thermal expansion lead to material ablation faster than the speed of sound and most of the deposited laser energy is released as kinetic energy in the ablation process. IHD only occurs when the exciting laser pulses have pulse durations shorter than the expansion time of the irradiated volume, which is on the sub-ns time scale for resonant mid-IR pulses. The challenge for practical application of this technology is to develop robust, compact laser systems with the correct wavelength and pulse durations to meet this condition.
Of prime importance is the generation of high peak power IR that is resonant with specific vibrational modes of water. The vibrational modes of water (OH stretch, OH bend and combinations) are the key to selectively deposit energy for laser cutting of biological materials. The lifetime of these vibrations are less than 200 fs so the laser deposited energy will essentially track the laser pulse time profile. In addition, the absorption is so strong that 90% of the IR tuned to the OH vibration is absorbed in a thickness of less than 1 μm. However, many other molecular vibrations are sufficiently short lived to satisfy the condition of IHD and most materials have a sufficiently high number density of at least one vibrational mode to ensure strong localization of the laser deposited energy. Mid-IR laser sources with the necessary pulse characteristics required for this application of IHD are currently too complex and costly to be of practical utility.
High energy, sub-ns mid-IR pulses were first generated by Free Electron Lasers. These devices are confined to large facilities and thus the expense and size of such systems limits their practical use. The lack of compact and simple laser sources in the mid-IR wavelength regions is due to the scarce choice of resonant laser gain media in this wavelength range. The few gain materials that are capable of resonant gain in the mid-IR, such as Er:YAG, Cr:YAG, etc. are either unsuitable for high power operation, or lack the necessary bandwidth to make picosecond (ps) pulses.
A number of methods using non-resonant nonlinear processes have been previously developed for generating sub-ns pulses in the mid-IR. However, until the invention disclosed herein all have suffered from drawbacks and are not suited for practical medical applications due to either their complexity or low efficiency and power.
There are at least three methods that involve the use of amplified Ti:Sapphire femtosecond pulses to produce broadband sub-ns mid-IR pulses. The first is continuum seeded difference frequency mixing, in which ultrafast mid-IR pulses are generated in a multi step process where short pulses at wavelengths between 1-2 μm are produced by an OPA, and the resulting signal and idler beams are then combined to produce pulses in 2-15 μm spectral region by difference frequency generation (DFG) [3, 4]. The second is the technique of an OPA with narrowband near-IR seed pulses [5]. The third reaches mid-IR wavelengths using an Optical Parametric Oscillators (OPO) pumped by Ti:Sapphire femtosecond pulses [2]. However the direct output of OPO's cannot produce the high energy picosecond pulses that are needed for these applications without subsequent amplification in an optical parametric amplifier (OPA).
All of these methods use Ti:Sapphire femtosecond pumps, which results in expensive, large, and complex systems which are not suitable as robust tools for use outside of the research environment. They involve no fewer than 6 individually complex subsystems requiring 10-μm sensitive alignment precision of at least 100 optomechanical mounts. The cost of these subsystems, and the prospect of keeping them all aligned to the required precision has prohibited the entry of such technology into general practice in all but a few applications. These Ti:Sapphire based schemes are also energetically inefficient, leading to relatively low output powers at mid-IR wavelengths.
High power mid-IR pulses can also be derived from picosecond solid state IR sources. In many such systems, the mid-IR seed pulses originate from optical parametric generation (OPG), which has a number of problems, including angular dispersion, low conversion efficiency and short coherence lengths [6]. The simplest method to date capable of generating tunable sub-ns picosecond mid-IR pulses requires a solid state laser oscillator, an OPO based on periodically poled crystals (for generating tunable near-IR picosecond seed pulses from the oscillator pulses) and a regenerative amplifier to amplify the oscillator pulses to high energy pump pulses [7]. Subsequent mixing to longer wavelengths was demonstrated in such a system with the use of additional OPA stages [8]. This method also suffers from high complexity due to the large number of complicated sub-systems required.
The method of seeding an OPA with ultrafast pulses that have been chirped for longer pulse durations is referred to as Optical Parametric Chirped Pulse Amplification (OPCPA). Such systems pumped by narrowband 1 μm pulses and seeded with chirped ultrashort Er:fibre 1.5 μm pulses, [9] are capable of generating broadband sub-ns pulses in the mid-IR. However, these previously implemented OPCPA systems also require many complicated and expensive sub-systems, limiting their use in medical and dental applications.
Despite the numerous attempts and the numerous methods available, no practical devices have been proposed or realized that can efficiently produce sub-ns pulses with repetition rates (<1 MHz), capable of supporting high pulse energy (>10 μJ) and spectrums in the wavelength region of 2-15 μm that are sufficiently low cost and robust for application in the medical and dental fields. We present a solution to this problem using OPA techniques to develop a novel method and laser apparatus capable of efficient generation of high-power sub-nanosecond pulses with controllable wavelengths in the 2 to 15 μm spectral region with a compact and robust design.